Isotopes of samarium

Naturally occurring samarium (₆₂Sm) is composed of five stable isotopes, ¹⁴⁴Sm, ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵²Sm and ¹⁵⁴Sm, and two extremely long-lived radioisotopes, ¹⁴⁷Sm (half life: 1.06×10¹¹ y) and ¹⁴⁸Sm (6.3×10¹⁵ y), with ¹⁵²Sm being the most abundant (26.75% natural abundance). ¹⁴⁶Sm is also fairly long-lived, but is not long-lived enough to have survived in significant quantities from the formation of the Solar System on Earth, although it remains useful in radiometric dating in the Solar System as an extinct radionuclide. A 2012 paper revising the estimated half-life of ¹⁴⁶Sm from 10.3(5)×10⁷ y to 6.8(7)×10⁷ y was retracted in 2023. It is the longest-lived nuclide that has not yet been confirmed to be primordial.

Other than the naturally occurring isotopes, the longest-lived radioisotopes are ¹⁵¹Sm, which has a half-life of 94.6 years, and ¹⁴⁵Sm, which has a half-life of 340 days. All of the remaining radioisotopes, which range from ¹²⁹Sm to ¹⁶⁸Sm, have half-lives that are less than two days, and the majority of these have half-lives that are less than 48 seconds. This element also has twelve known isomers with the most stable being ^141mSm (t_1/2 22.6 minutes), ^143m1Sm (t_1/2 66 seconds) and ^139mSm (t_1/2 10.7 seconds).

The long lived isotopes, ¹⁴⁶Sm, ¹⁴⁷Sm, and ¹⁴⁸Sm, primarily decay by alpha decay to isotopes of neodymium. Lighter unstable isotopes of samarium primarily decay by electron capture to isotopes of promethium, while heavier ones decay by beta decay to isotopes of europium.

Isotopes of samarium are used in samarium–neodymium dating for determining the age relationships of rocks and meteorites.

¹⁵¹Sm is a medium-lived fission product and acts as a neutron poison in the nuclear fuel cycle. The stable fission product ¹⁴⁹Sm is also a neutron poison.

Samarium is theoretically the lightest element with even atomic number with no stable isotopes (all isotopes of it can theoretically go either alpha decay or beta decay or double beta decay), other such elements are those with atomic numbers > 66 (dysprosium, which is the heaviest theoretically stable nuclide).